The present invention relates to a non-Cu-based cast Al alloy that contains substantially no Cu, and to a method for the heat treatment thereof.
Alxe2x80x94Si-based aluminum (Al) alloys that mainly consist of Al containing several percent Si by weight have been known as Al alloys for casting or die casting, and multi-phase Alxe2x80x94Si-based alloys that further contain other elements such as Cu and Mg in addition to the basic composition of the Alxe2x80x94Si-based Al alloys have been used as alloys for casting. This is because these alloys have better properties important for casting or die casting, such as the fluidity of the molten metal and mold filling, than other alloys; little casting crack occurs; alloys that have higher strength can be obtained by combining other elements; and these alloys have a small coefficient of thermal expansion and a high wear resistance.
Examples of Alxe2x80x94Si-based alloys to which a small quantity of Mg is added include AC4A, AC4C, and AC4CH. These alloys have increased strength by the effect of heat treatment to precipitate the intermediate phase of Mg2Si. In particular, AC4C and AC4CH, which has increased toughness by limiting the Fe content to 0.20% by mass or less, is used as alloys for wheels of vehicles such as motor vehicles.
Furthermore, Alxe2x80x94Si-based alloys to which small quantities of Mg and Cu are added are also used. The strength of these alloys is improved by precipitation hardening by the intermediate phase of Mg2Si, solid solution hardening by Cu, and precipitation hardening by the intermediate phase of Al2Cu.
As described above, the improvement of strength of heat-treated Al alloys is achieved by the addition of other elements, and the aging and precipitation of resultant intermediate phases, and the heat treatment for aging and precipitation consists of the solution treatment and the aging treatment. The solution treatment is a heat treatment for obtaining the solid solution of a uniform composition at a high temperature, by adding a non-equilibrium phase crystallized during solidification to form a solid solution, and further adding a precipitated phase during cooling to form the solid solution. The aging treatment carried out following the solution treatment is a heat treatment for making the precipitated intermediate phases fine and uniform, and for causing precipitation hardening with the precipitated intermediate phases. These heat treatments improve the mechanical properties of the Al alloy.
Heretofore, although a controlled atmosphere furnace that uses the air as the heat medium, such as a tunnel furnace, has been used for the solution treatment and the aging treatment of such Al alloys, heating-up takes a long time, and the deviation of temperature is as large as about xc2x15xc2x0 C., resulting in the difficulty of the solution treatment at a higher temperature.
Although above-described Alxe2x80x94Si-based alloys to which various elements such as Mg and Cu are added have been used as the Al alloys as the mechanical properties, they have a tensile strength of about 290 MPa, a 0.2% yield strength of about 200 MPa, and an elongation of about 8%. In an Al alloy used for wheels of motor vehicles, it is advantageous to improve the mechanical properties such as tensile strength, 0.2% yield strength, and elongation, to make the wheels of motor vehicles thinner, thereby reducing the entire weight of the motor vehicle, decreasing rolling resistance, and contributing to the elevation of fuel consumption and exhaust gas purification performance, as well as the improvement of driving stability.
On the other hand, although the strength of an Al alloy has been improved by adding Cu to the Al alloy, as described above, the corrosion resistance of the Al alloy lowers if more than a predetermined quantity of Cu is added.
Considering the above-described problems in conventional Al alloys, an object of the present invention is to provide a non-Cu-based cast Al alloy that has well-balanced mechanical properties of tensile strength, yield strength, and elongation.
Another object of the present invention is to provide a method for the heat treatment of a non-Cu-based cast Al alloy that can perform the solution treatment at an increased speed of heating-up time, with small deviation of temperature, and at a higher temperature.
According to the present invention, there is provided a non-Cu-based cast Al alloy that contains substantially no Cu, wherein the tensile strength thereof is 305 MPa or more, the yield strength thereof is 220 MPa or more, and the elongation thereof is 10% or more.
Also, the Al alloy of the present invention preferably contains 6.5 to 7.5% by mass of Si, and 0.36% by mass or less Mg, and more preferably contains 20 to 70 ppm of Sr. It is also preferable that the Al alloy of the present invention is a precipitation hardened alloy. Such a non-Cu-based cast Al alloy can be suited to wheels of vehicles such as motor vehicles.
Also, according to the present invention, there is provided a method for heat treatment of a cast Al alloy comprising; subjecting a work piece of the cast Al alloy to solution treatment, and then subjecting the work piece to aging treatment, to improve mechanical properties of the work piece, wherein at least said solution treatment is performed by rapidly raising the temperature to the solution treatment temperature within 30 minutes, and maintaining said solution treatment temperature within 3 hours to form the non-Cu-based cast Al alloy that has a tensile strength of 305 MPa or more, a 0.2% yield strength of 220 MPa or more, and an elongation of 10% or more.
Furthermore, according to the present invention, there is provided a method for heat treatment of a cast Al alloy comprising; subjecting a work piece of the cast Al alloy to solution treatment, and then subjecting the work piece to aging treatment, to improve mechanical properties of the work piece, wherein at least said solution treatment is performed by allowing said work piece to be present in a fluidized bed to form the non-Cu-based cast Al alloy that has a tensile strength of 305 MPa or more, a 0.2% yield strength of 220 MPa or more, and an elongation of 10% or more.
In the heat treatment method of the present invention, the above-described aging treatment is preferably performed by allowing the work piece to be present in a fluidized bed. Also, the fluidized bed is preferably formed by the direct blowing of hot air.